Don't go marrying your cousin quite yet

I just read the Decode paper on reproductive success and relatedness (here. You can read other comments on it at Gene Expression here and here , and over at John Hawks. Overall I think it is a really interesting finding. One of the impressive things is that the number of grandchildren that a couple produced is still a decreasing function of relatedness out to 5 and 6 cousins. As the authors note this is pretty convincing evidence that this relationship is not due to a conscious choice by couples: (as ' Relationships at this genealogical distance are rarely known to the couples or their families and acquaintances in their social environment'), and so they conclude that this phenomenon is probably biological.

However I wonder whether there could be a non-conscious non-biological forces that could potentially give rise to this phenomenon. For example, in small communities there are fewer people to choose a partner from, so you are more likely to chose a relative (just by accident). In larger towns you have more choice and so are likely to pick some one less related to you (just by chance). If the number of children born per family is higher in smaller communities then partners with close relatedness will have more children will have more children purely because there are more of them in small communities. Does this make sense, or have I missed something? The authors adjust for spatial component in their model by putting the location of the couple (one of 21 counties) in as a fixed effect in the model. But this level of spatial structure might be crude compared to the information needed to detect an effect of town size. It is also a shame that they don't have marriage dates for the couples, as it would be interesting to know if the relationship between family size and relatedness is due to marrying earlier or a higher birthrate.

If this phenomenon has a biological basis that would be really interesting, and as noted elsewhere it could be due to subtly higher attractiveness between closer relatives (obviously not too close) or a high birthrate due to reduced genetic incompatibilities between mother and child. Even if it is not biological it is an interesting observation. It would also be great to see it replicated in different human populations. I wonder if this work could be replicated in macaque or some other model organism. I look forward to seeing more papers looking at this.